Semiconductor processing often involves etching of wafers or substrates. A substrate surface may include both an exposed nitride layer and an exposed oxide layer in different regions of a wafer. A nitride-containing film may line a bottom surface and sidewalls of a semiconductor device feature, such as a recess. An oxide film may be deposited on portions of the nitride film. A device feature may have a high aspect ratio (i.e., where a horizontal opening is relatively small compared to the depth of the device feature) or a low aspect ratio (i.e., where a horizontal opening is relatively large compared to the depth of the feature). When more than one film is present on the surface of a substrate, non-selective etching of one film may involve etching another; however, etching of a particular film may or may not be desirable in some circumstances. Controlling relative etch rates of one material, as compared to another material, permits selective etching of one film in some circumstances, and etching of all exposed film surfaces in other circumstances.
Fabrication of a semiconductor device may involve hundreds of steps, many of which correspond to etching processes. In some settings, it may be desirable to have a very low or a very high etch rate. A very low etch rate, however, may not be commercially desirable to the extent that manufacturing throughput may be reduced. Consequently, semiconductor processes employ etching selectivities and absolute etch rates that are selected to maximize manufacturing throughput while minimize undesired etching.